1. Field of the Invention
The present invention relates to an angular rate sensor used for detecting e.g., video camera shake, the operation of a virtual reality apparatus, the direction in a car navigation system or the like.
2. Description of the Related Art
A so-called oscillation gyro type angular rate sensor has been widely available as consumer use. The oscillation gyro type angular rate sensor detects angular rate by oscillating a rod-like oscillator at a predetermined resonance frequency and detecting Coriolis force generated by influence of angular rate with a piezoelectric element or the like.
For driving an oscillator in the angular rate sensor like this, a method using a separately-excited oscillation type driving circuit and one using a self-excited oscillation type driving circuit are available. However, the method using a separately-excited oscillation type driving circuit has a problem that when a difference is made between oscillation frequency and resonance frequency of an oscillator due to influence of temperature characteristics of the oscillator or the like, sensitivity for detecting Coriolis force rapidly decreases. Therefore, the method using a separately-excited oscillation type driving circuit has not been in practical use.
Consequently, the method using a self-excited oscillation type driving circuit in which the oscillator is incorporated in a loop of a phase-shift oscillator circuit is now widely used. Since the angular rate sensor using this method self-oscillates at the resonance frequency of oscillator, sensitivity thereof hardly changes due to influence of the temperature characteristics, thereby obtaining an angular rate output having stable sensitivity in a wide temperature range (refer, for example, to Jpn. Pat. Appln. Laid-pen Publication No. 2000-131077).
A conventional angular rate sensor shown in FIGS. 1A, 1B, and 1C includes a triangular prism-like oscillator 104 having a triangular prism-like constant elastic oscillator 100. The constant elastic oscillator 100 has first to third piezoelectric elements 101 to 103 attached to the side surfaces thereof, respectively. The first piezoelectric element 101 is composed of an electrode 101a and a piezoelectric material 101b. The second piezoelectric element 102 is composed of an electrode 102a and a piezoelectric material 102b. The third piezoelectric element 103 is composed of an electrode 103a and a piezoelectric material 103b. For example, the constant elastic oscillator 100 is a constant elastic metal oscillator.
The conventional angular rate sensor includes an amplifier 105 connected to the first piezoelectric element 101, a phase shifter 106 connected to the amplifier 105, a differential amplifier 107 connected to the second and third piezoelectric elements 102 and 103, a synchronous detector 108 connected to the differential amplifier 107, and a low-pass filter 109 connected to the synchronous detector 108. In the conventional angular rate sensor, the second and third piezoelectric elements 102 and 103 detect oscillation of the oscillator 104 for performing self-excited oscillation as well as Coriolis force generated in the oscillator 104.
The angular rate sensor using the triangular prism-like oscillator 104 has the highest sensitivity at the present time and therefore is currently mainstream. However, the angular rate sensor of this type has a complicated structure, which makes it difficult to produce high volume efficiency in manufacturing process. For example, in the above configuration, process of bonding piezoelectric elements to each of the triangular prism-like constant elastic oscillators is required, with the result that volume efficiency cannot be improved. Further, along with the miniaturization of the sensor, accuracy in a support mechanism or bonding accuracy of the piezoelectric element to the constant elastic metal oscillator has been increasingly demanded. In addition, influence of a bonding layer to the oscillator is increased. Therefore, manufacturing efficiency is lowered and manufacturing cost is significantly increased.
Another conventional angular rate sensor shown in FIGS. 2A, 2B and 2C includes an oscillator 117 having a columnar piezoelectric ceramic oscillator 110. The piezoelectric ceramic oscillator 110 has six electrodes 111 to 116 printed on a side surface thereof. The first to third electrodes 111 to 113 are independently formed. The fourth to sixth electrodes 114 to 116 are connected to the same ground potential. This angular rate sensor includes an amplifier 118 connected to the first electrode 111, a phase shifter 119 connected to the amplifier 118, an adder 120 connected to the phase shifter 119, a differential amplifier 121 connected to the second and third electrodes 112 and 113, a synchronous detector 122 connected to the differential amplifier 121, and a low-pass filter 123 connected to the synchronous detector 122. This angular rate sensor applies a voltage to the first electrode 111 to oscillate the oscillator 117, and detects Coriolis force generated in the oscillator 117 with the second and third electrodes 112 and 113.
In this conventional angular rate sensor, the electrodes 111 to 116 are printed on the oscillator 117 as described above. This eliminates the need to bond the piezoelectric elements to the oscillator 117 and makes the structure of the sensor relatively simple. However, in the case where the sensor size is reduced, it is difficult to produce an accurately configured piezoelectric ceramic oscillator 110 and, it is also difficult to print the electrodes onto the piezoelectric ceramic oscillator 110 with high accuracy.
That is, while this conventional angular rate sensor uses the columnar piezoelectric ceramic oscillator 110, it is difficult to manufacture, with high accuracy, the columnar piezoelectric ceramic oscillator 110 as compared to the triangular prism-like or quadratic prism like oscillator. Further, it is not easy to print with high accuracy the electrodes onto the rounded surface of this angular rate sensor. As described above, the use of the columnar piezoelectric ceramic oscillator 110 makes it difficult to produce the angular rate sensor in large volume. Even though the mass-production has been realized, it is difficult to reduce manufacturing cost.
Still another conventional angular rate sensor 201 shown in FIG. 3 includes: a quadratic prism-like oscillator 201a; an electrode 204 formed on a side surface 202a of a ferrite section 202; and a piezoelectric element 203. The quadratic prism-like oscillator 201a is formed by laminating the ferrite section 202 and a piezoelectric ceramic section 203a which is a piezoelectric material. The piezoelectric element 203, which faces the electrode 204 across the oscillator 201a, is constituted by electrodes 203b and 203c disposed on a side surface 203a1 of the piezoelectric ceramic section 203a and the piezoelectric ceramic section 203a. The angular rate sensor 201 applies a voltage between the electrode 204 and electrodes 203b and 203c to allow the piezoelectric element 203 to oscillate the oscillator 201a. Further, the angular rate sensor 201 includes an adder 210, an amplifier 211, and a phase-shifter 212. These components 210 to 212 and electrodes 203b and 203c of the piezoelectric element 203 function as an oscillation drive section for oscillating the oscillator 201a. Further, the angular rate sensor 201 includes a differential amplifier 213, a synchronous detector 214 connected to the output of the adder 210, and a low-pass filter 215. These components 213 to 215 and outside electrodes 203b and 203c of the piezoelectric element 203 function as a detection section for detecting oscillation of the ferrite section 202. That is, the piezoelectric element 203 of the angular rate sensor 201 has a function of driving oscillation as well as a function of detecting the oscillation. With the above configuration, the angular rate sensor 201 detects Coriolis force generated in the oscillator 201a with the piezoelectric element 203, thereby detecting angular rate.
It is only necessary to provide a piezoelectric element only on one surface in this angular rate sensor, which makes the structure thereof relatively simple. However, in this conventional angular rate sensor, electrodes function as drive electrodes as well as detection electrodes. It follows that, when driving efficiency is adjusted to a desired value according to the shape or size of the drive electrode, detection efficiency also changes. Thus, it is difficult to adjust the driving and detection efficiency to desired values at the same time.
That is, the conventional angular rate sensor shown in FIGS. 1A, 1B and 1C has a complicated structure due to the need of bonding the piezoelectric element to the constant elastic metal oscillator, and a complicated oscillator support mechanism. The conventional angular rate sensor shown in FIGS. 2A, 2B and 2C uses the columnar piezoelectric ceramic oscillator, which makes it difficult to manufacture the oscillator with high accuracy. Further, it is not easy to print electrodes onto the curved surface accurately.